GURPS Creatures of the Night, Vol. 5
W23!

The darkness lurks, waiting, under the sea. It hides in riverbeds and lakes, judging its victims. Even the streets of civilization are not safe from perilous forces that pervert the expected. Beware.

In this installment of Creatures of the Night, the elements of evil hide in plain sight, corrupting the familiar and turning seemingly safe situations into reason for panic. Crayfen, moon guires, and gully dragons pose a danger to those who venture near water; dreadstalks wet their vegetative fangs in anticipation of victims; and stealth golems and watchtowers occupy spaces where humanity thinks itself safe. These six examples of strangeness all contain information for GURPS, including adventure ideas and possibilities for altering the adversaries for different situations.

Don't bother keeping your eyes open for danger; you probably won't be able to see it anyway.